In the management of diabetes, the regular measurement of glucose in the blood is essential in order to ensure correct insulin dosing. Furthermore, it has been demonstrated that in the long term care of the diabetic patient better control of the blood glucose levels can delay, if not prevent, the onset of retinopathy, circulatory problems and other degenerative diseases often associated with diabetes. Thus there is a need for reliable and accurate self-monitoring of blood glucose levels by diabetic patients.
Currently, blood glucose is monitored by diabetic patients with the use of commercially available calorimetric test strips or electrochemical biosensors (e.g. enzyme electrodes), both of which require the regular use of a lancet-type instrument to withdraw a suitable amount of blood each time a measurement is made. On average, the majority of diabetic patients would use such instruments to take a measurement of blood glucose twice a day. However, the US National Institutes of Health recently recommended that blood glucose testing should be carried out at least four times a day, a recommendation that has been endorsed by the American Diabetes Association. This increase in the frequency of blood glucose testing imposes a considerable burden on the diabetic patient, both in terms of financial cost and in terms of pain and discomfort, particularly in the long-term diabetic who has to make regular use of a lancet to draw blood from the fingertips. Thus, there is clearly a need for a better long-term glucose monitoring system that does not involve drawing blood from the patient.
There have been a number of recent proposals for glucose measurement techniques that do not require blood to be withdrawn from the patient. Various attempts have been made to construct devices in which an enzyme electrode biosensor is placed on the end of a needle or catheter which is inserted into a blood vessel (Wilkins, E. and Atanasov, P, Med. Eng. Phys (1996) 18: 273–288). Whilst the sensing device itself is located within a blood vessel, the needle or catheter retains connection to the external environment. In practice, such devices are not suitable for use in human patients first because the insertion of a needle or catheter into a blood vessel poses an infection risk and is also uncomfortable for the patient and hence not suitable for continuous use. Secondly, devices of this type have not gained approval for use in patients because it has been suggested that the device itself, on the end of a needle or catheter, may be responsible for the shedding of thromboses into the patient's circulation. This obviously poses a very serious risk to the patient's health.
Mansouri and Schultz (Biotechnology 1984), Meadows and Schultz (Anal. Chim. Acta. (1993) 280: pp 21–30) and U.S. Pat. No. 4,344,438 all describe devices for the in situ monitoring of low molecular weight compounds in the blood by optical means. These devices are designed to be inserted into a blood vessel or placed subcutaneously but require fibre-optic connection to an external light source and an external detector. Again the location of these devices in a blood vessel carries an associated risk of promoting thromboses and in addition, in one embodiment the need to retain a fibre-optic connection to the external environment is impractical for long-term use and carries a risk of infection.
In the search for a less invasive glucose monitoring technique some attention has also been focussed on the use of infra-red spectroscopy to directly measure blood glucose concentration in blood vessels in tissues such as the ear lobe or finger tip which are relatively “light transparent” and have blood vessels sited close to the surface of the skin (Jaremko, J. and Rorstad, O. Diabetes Care 1998 21: 444–450 and Fogt, E. J. Clin. Chem. (1990) 36: 1573–80). This approach is obviously minimally invasive, but has proven to be of little practical value due to the fact that the infra-red spectrum of glucose in blood is so similar to that of the surrounding tissue that in practical terms it is virtually impossible to resolve the two spectra.
It has been observed that the concentration of analytes in subcutaneous fluid correlates with the concentration of said analytes in the blood, and consequently there have been several reports of the use of glucose monitoring devices which are sited in a subcutaneous location. In particular, Atanasov et al. (Med. Eng. Phys. (1996) 18: pp 632–640) describe the use of an implantable glucose sensing device (dimensions 5.0×7.0×1.5 cm) to monitor glucose in the subcutaneous fluid of a dog. The device consists of an amperometric glucose sensor, a miniature potentiostat, an FM signal transmitter and a power supply and can be interrogated remotely, via antenna and receiver linked to a computer-based data acquisition system, with no need for a connection to the external environment. However, the large dimensions of this device would obviously make it impractical for use in a human patient.
Ryan J. Russell et al, Analytical Chemistry, Vol. 71, Number 15, 3126–3132 describes an implantable hydrogel based on polyethyleneglycol containing fluorescein isothiocyanate dextran (FITC-dextran) and tetramethylrhodamine isothiocyanate concavalin A chemically conjugated to the hydrogel network for dermal implantation. The implanted hydrogel spheres are to be transdermally interrogated.
R. Ballerstadt et al, Analytica Chemica Acta, 345 (1997), 203–212 discloses an assay system in which two polymer (dextran) molecules are respectively labelled with first and second fluorophores and are bound together by multivalent lectin molecules, producing quenching. Glucose saturates the binding sites of the lectin, causing disassociation of the two polymers, giving an increase in fluorescence.
Joseph R. Lakowicz et al, Analytica Chimica Acta, 271, (1993), 155–164 describes the use of phase modulation fluorimetry. This substitutes a fluorescence lifetime based measurement for the fluorescence intensity based measurements taught in the earlier described art.
Fluorescence lifetime can be measured by a phase modulation technique by exciting fluorescence using light which is intensity modulated at 1 to 200 MHz and measuring the phase shift of the emission relative to the incident light and the modulation of the emission.
In WO91/09312 a subcutaneous method and device is described that employs an affinity assay for glucose that is interrogated remotely by optical means. In WO97/19188 a further example of an implantable assay system for glucose is described which produces an optical signal that can be read remotely. The devices described in WO91/09312 and WO97/19188 will persist in the body for extended periods after the assay chemistry has failed to operate correctly and this is a major disadvantage for chronic applications. Removal of the devices will require a surgical procedure.
WO00/02048 deals with this problem by using a biodegradable material to contain the assay reagents. There the assay materials would be likely to be in contact with the bloodstream once the biodegradable material has degraded. It would be desirable to minimise or avoid this.
There remains a clear need for sensitive and accurate blood glucose monitoring techniques which do not require the regular withdrawal of blood from the patient, which do not carry a risk of infection or discomfort and which do not suffer from the practical disadvantages of the previously described implantable devices.